Abstract: A cache coherency controller comprises a directory indicating, for memory addresses cached by a group of two or more cache memories in a coherent cache structure, which of the cache memories are caching those memory addresses, the directory being associative so that multiple memory addresses map to an associative set of more than one directory entry; and control logic responsive to a memory address to be newly cached, and configured to detect whether one or more of the set of directory entries mapped to that memory address is available for storage of an indication of which of the two or more cache memories are caching that memory address; the control logic being configured so that when all of the set of directory entries mapped to that memory address are occupied, the control logic is configured to select one of the set of directory entries as a directory entry to be overwritten and the corresponding cached information to be invalidated, the control logic being configured to select a directory entry to be ove

Abstract: An interconnect, and method of operation of an interconnect, are provided for connecting a plurality of master devices and a plurality of slave devices. Hazard management circuitry is used to serialize transactions to overlapping addresses. In addition, gating circuitry ensures ordered write observation (OWO) behavior at an interface to one or more of the master devices, the gating circuitry receiving write address transfers of write transactions and performing a gating operation to gate onward propagation of the write address transfers to the slave devices in order to ensure the OWO behavior. The gating circuitry performs the gating operation under control of the hazard management circuitry. Hence, for write transactions that are subjected to hazard checking by the hazard management circuitry, this removes the need to implement any other processes to specifically manage OWO behavior for those write transactions.

Abstract: Interconnect circuitry for connecting transaction masters to transaction slaves includes response modification circuitry. The response modification circuitry includes shortlist buffer circuitry storing identification for modification target transaction responses. The response modification circuitry uses this identification data to identify among a stream of transaction responses in transit a modification target transaction response. The response modification circuitry then serves to form a modified transaction response to be sent in place of the modification target transaction response to the transaction master.

Abstract: A system-on-check integrated circuit 2 includes interconnect circuitry 4 connecting a plurality of transaction sources to a plurality of transaction destinations. The interconnect circuitry 4 includes a reorder buffer for buffering access transactions and hazard checking circuitry 46, 48, 50, 52 for performing hazard checks, such as point-of-serialization checks and identifier reuse checks. Check suppression circuitry 62, 64, 66, 68 serves to suppress one or more hazard checks depending upon one or more state variables that themselves depend upon access transactions other than the access transaction for which the hazard checking is or is not to be suppressed. As an example, hazard checking may be suppressed if it is known that there are no other access transactions currently buffered within the reorder buffer 26 or alternatively no other access transactions from the same transaction source buffered within the reorder buffer 26.

Abstract: An interconnect and method of managing a snoop filter within such an interconnect are provided. The interconnect is used to connect a plurality of devices, including a plurality of master devices where one or more of the master devices has an associated cache storage. The interconnect comprises coherency control circuitry to perform coherency control operations for data access transactions received by the interconnect from the master devices. In performing those operations, the coherency control circuitry has access to snoop filter circuitry that maintains address-dependent caching indication data, and is responsive to a data access transaction specifying a target address to produce snoop control data providing an indication of which master devices have cached data for the target address in their associated cache storage.

Abstract: An interconnect has coherency control circuitry for performing coherency control operations and a snoop filter for identifying which devices coupled to the interconnect have cached data from a given address. When an address is looked up in the snoop filter and misses, and there is no spare snoop filter entry available, then the snoop filter selects a victim entry corresponding to a victim address, and issues an invalidate transaction for invalidating locally cached copies of the data identified by the victim. The coherency control circuitry for performing coherency checking operations for data access transactions is reused for performing coherency control operations for the invalidate transaction issued by the snoop filter. This greatly reduces the circuitry complexity of the snoop filter.

Abstract: A cache coherency controller comprises a directory indicating, for memory addresses cached by a group of two or more cache memories in a coherent cache structure, which of the cache memories are caching those memory addresses, the directory being associative so that multiple memory addresses map to an associative set of more than one directory entry; and control logic responsive to a memory address to be newly cached, and configured to detect whether one or more of the set of directory entries mapped to that memory address is available for storage of an indication of which of the two or more cache memories are caching that memory address; the control logic being configured so that when all of the set of directory entries mapped to that memory address are occupied, the control logic is configured to select one of the set of directory entries as a directory entry to be overwritten and the corresponding cached information to be invalidated, the control logic being configured to select a directory entry to be ove

Abstract: A cache coherency controller comprises a directory indicating, for memory addresses cached by one or more of a group of one or more cache memories connectable in a coherent cache structure, which of the cache memories are caching those memory addresses; and control circuitry configured to detect a directory entry relating to a memory address to be accessed so as to coordinate, amongst the cache memories, an access to a memory address by one of the cache memories or a coherent agent in instances when the directory entry indicates that another of the cache memories is caching that memory address; the control circuitry being responsive to status data indicating whether each cache memory in the group is currently subject to cache coherency control so as to take into account, in the detection of the directory entry relating to the memory address to be accessed, only those cache memories in the group which are currently subject to cache coherency control.

Abstract: An interconnect has coherency control circuitry for performing coherency control operations and a snoop filter for identifying which devices coupled to the interconnect have cached data from a given address. When an address is looked up in the snoop filter and misses, and there is no spare snoop filter entry available, then the snoop filter selects a victim entry corresponding to a victim address, and issues an invalidate transaction for invalidating locally cached copies of the data identified by the victim. The coherency control circuitry for performing coherency checking operations for data access transactions is reused for performing coherency control operations for the invalidate transaction issued by the snoop filter. This greatly reduces the circuitry complexity of the snoop filter.

Abstract: There is provided an interconnect for transferring requests between ports in which the ports include both source ports destination ports. The interconnect includes storage circuitry for storing the requests. Input circuitry receives the requests from the plurality of source ports, selects at least one selected source port from an allowed set of said plurality of source ports, and transfers a presented request from the at least one selected source port to the storage circuitry. Output circuitry causes a request in said storage circuitry to be output at one of said plurality of destination ports. Counter circuitry maintains counter values for a plurality of tracked ports from amongst said ports, each counter value indicating the number of requests in said storage circuitry associated with a corresponding tracked port that are waiting to be output by said output circuitry and filter circuitry determines whether or not a given source port is in said allowed set in dependence on said counter circuitry.

Abstract: An interconnect, and method of operation of an interconnect, are provided for connecting a plurality of master devices and a plurality of slave devices. Hazard management circuitry is used to serialise transactions to overlapping addresses. In addition, gating circuitry ensures ordered write observation (OWO) behaviour at an interface to one or more of the master devices, the gating circuitry receiving write address transfers of write transactions and performing a gating operation to gate onward propagation of the write address transfers to the slave devices in order to ensure the OWO behaviour. The gating circuitry performs the gating operation under control of the hazard management circuitry. Hence, for write transactions that are subjected to hazard checking by the hazard management circuitry, this removes the need to implement any other processes to specifically manage OWO behaviour for those write transactions.

Abstract: A data array has multiple ways, each way having entries for storing data values. In response to a write request, an updated data value having a target address may be stored in any of a corresponding set of entries comprising an entry selected from each way based on the target address. An update queue stores update information representing pending write requests. Update information is selected from the update queue for a group of pending write requests corresponding to different ways, and these write requests are performed in parallel so that updated values are written to entries of different ways.

Abstract: Interconnect circuitry 10 for connecting transaction masters 4, 6, 8 to transaction slaves 12, 14 includes response modification circuitry 18. The response modification circuitry includes shortlist buffer circuitry 28 storing identification for modification target transaction responses. The response modification circuitry 18 uses this identification data to identify among a stream of transaction responses in transit a modification target transaction response. The response modification circuitry 18 then serves to form a modified transaction response to be sent in place of the modification target transaction response to the transaction master 4, 6, 8.

Abstract: An interconnect has transaction tracking circuitry for enforcing ordering of a set of data access transactions so that they are issued to slave devices in an order in which they are received from master devices. The transaction tracking circuitry is reused for also enforcing ordering of snoop transactions which are triggered by the set of data access transactions, for snooping master devices identified by a snoop filter as holding cache data for the target address of the transactions.

Abstract: An arbiter is configured to select one of several contending data packets transmitted from an initiator, the data packets comprising an identifier identifying the initiator and data. The arbiter comprises: a history buffer for storing the identifiers identifying the initiators of a plurality of recently selected data packets; and selection circuitry configured to select one of the contending data packets in dependence upon the initiators of the contending data packets and the initiators identified in the history buffer, such that a probability of a data packet being selected increases with the number of data packets selected since a data packet from the same initiator was selected.

Abstract: An interconnect has coherency control circuitry for performing coherency control operations and a snoop filter for identifying which devices coupled to the interconnect have cached data from a given address. When an address is looked up in the snoop filter and misses, and there is no spare snoop filter entry available, then the snoop filter selects a victim entry corresponding to a victim address, and issues an invalidate transaction for invalidating locally cached copies of the data identified by the victim. The coherency control circuitry for performing coherency checking operations for data access transactions is reused for performing coherency control operations for the invalidate transaction issued by the snoop filter. This greatly reduces the circuitry complexity of the snoop filter.

Abstract: An interconnect has coherency control circuitry for performing coherency control operations and a snoop filter for identifying which devices coupled to the interconnect have cached data from a given address. When an address is looked up in the snoop filter and misses, and there is no spare snoop filter entry available, then the snoop filter selects a victim entry corresponding to a victim address, and issues an invalidate transaction for invalidating locally cached copies of the data identified by the victim. The coherency control circuitry for performing coherency checking operations for data access transactions is reused for performing coherency control operations for the invalidate transaction issued by the snoop filter. This greatly reduces the circuitry complexity of the snoop filter.

Abstract: An interconnect and method of managing a snoop filter within such an interconnect are provided. The interconnect is used to connect a plurality of devices, including a plurality of master devices where one or more of the master devices has an associated cache storage. The interconnect comprises coherency control circuitry to perform coherency control operations for data access transactions received by the interconnect from the master devices. In performing those operations, the coherency control circuitry has access to snoop filter circuitry that maintains address-dependent caching indication data, and is responsive to a data access transaction specifying a target address to produce snoop control data providing an indication of which master devices have cached data for the target address in their associated cache storage.

Abstract: An interconnect has transaction tracking circuitry for enforcing ordering of a set of data access transactions so that they are issued to slave devices in an order in which they are received from master devices. The transaction tracking circuitry is reused for also enforcing ordering of snoop transactions which are triggered by the set of data access transactions, for snooping master devices identified by a snoop filter as holding cache data for the target address of the transactions.

Abstract: A data processing apparatus is provided with a master device and a slave device which communicate via communication circuitry. The slave device is associated with a predetermined number of permission tokens that is equal to a maximum number of currently pending messages that can be accepted for processing from the communication circuitry by that slave device. The slave device transmits these permission tokens to the master device. The master device takes exclusive temporary possession of the permission tokens that it receives such that the permission tokens are then no longer available to any other master device. A master device initiates a message to a slave device when the master device has exclusive temporary possession of a permission token for that slave device. When the master device has initiated its message, then it relinquishes the exclusive temporary possession of the permission token such that it is then available for other devices.